Selective call messaging, such as paging messaging, conventionally involves the use of a paging terminal which compiles paging messages for transmission from a transmitter as a radio frequency (RF) signal. The RF signal is then received by portable paging receivers which decode the signal to recover messages intended for reception by the paging receivers. In typical paging systems, the paging receivers comprise different types of receivers. For example, voice and tone paging receivers can be included in a system for receiving analog data, while alphanumeric and numeric receivers usually receive digital data. Therefore, the paging terminal must, in many systems, be capable of transmitting different types of signals.
Generally, the paging terminal receives paging information from a conventional telephone set coupled to the terminal by a telephone system. The terminal, for each set of information, refers to a subscriber database to determine the paging address of the targeted receiver and the type of message, i.e., voice or tone, that should be transmitted to the targeted receiver. In systems including both tone and voice pagers, the paging terminal has the ability to transmit both tone and voice messages. Conventionally, analog voice signals comprising paging messages are generated on first analog lines during certain time periods, while analog tone signals are generated on second analog lines during other time periods. The terminal does not usually indicate which of the two types of analog signals are generated during each of the times. The signals on these two lines are then scaled independently by a separate controller and summed to produce a single analog output from the controller. This analog output is next provided to the transmitter for transmission to the paging receivers. Because the tone and voice signals are produced at different times, the analog output can include either voice or tone signals, but not both, at any particular time. However, this type of analog message generation is not ideal because, during voice transmissions, noise on the tone lines will be summed with the voice signals and vice versa. As a result, the receivers targeted for reception of analog messages can receive noisy signals that may be difficult for the user to comprehend.
Thus, what is needed is a method and apparatus for reducing noise in analog signals produced by paging terminals which provide both tone and voice signals.